Pre-ceramic polymers

To obtain information on the reason for NMR signal broadening and insight into the chemical nature of these pre-ceramic polymers, attempts were made to follow the reaction of the sym-tetrachlorodisilane and HMDZ by ySi NMR spggtroscopy. 

The following focuses on the structural characterization of the pre-ceramic polymers. 

Cone Calorimeter Data for Polymers and Pre-Ceramic Polymer-Polymer Blends... 

Hydrosilylation of 1 with trichlorosilane has been performed with Pt on charcoal (1% by weight) in quantitative yields (Scheme 1). The B-tris(trichlorosilylvinyl)borazine (2) was obtained with a high regioselectivity of proximately 80% trans hydrosUylation product [4], Pure 2 can be obtained by fiactional crystallization of the synthesized product fiom hexane. For further synthesis, both a- and P-hydrosilylation products can be used. No further hydrosilylation was observed in this case. In order to interconnect the single source precursor molecule 2 to a pre-ceramic polymer, methylamine was added to the solution of 2 in hexane, and a high viscosity, colorless oil was formed. By changing the reaction parameters (excess of CH3NH2, temperature), the viscosity of the polymer can be varied [5]. The obtained polymer (3P) is pure after evaporation of the solvent, which is checked by NMR. Other solvents like thf or toluene are also possible for the reaction, as well as for dissolution of the polymer. Furthermore, ethylamine leads to similar results in the formation of the polymer. 

Polymer pyrolysis to form advanced ceramics allows the production of highly covalent refractory components (fibers, films, membranes, foams, joints, monolithic bodies, ceramic matrix composites) that are difficult to fabricate via the traditional powder processing route [1-4]. Yajima was the first to demonstrate the feasibility of producing high-strength SiC-based fibers from pyrolysis of polycarbosilane [5]. In this process, a thermoplastic pre-ceramic polymer is first shaped into the desired form, cross-linked into a pre-ceramic network and finally converted into a ceramic material by a pyrolysis process in a controlled atmosphere (Fig. 1). A common feature of the polymer route is the formation of intermediates called amorphous covalent ceramics (ACC) [6]. These are formed after removal of the organic components and before crystallization that occurs at higher temperatures. 

In order to overeome these difficulties, many hybrid processes have been studied for the fabrication of bulk ceramic components via polymer pyrolysis. Greil and coworkers proposed to reduce the shrinkage by the use of active fillers that can react during pyrolysis leading to a volume expansion [33], pre-ceramic polymers... 

Figure 7. CP MAS NMR spectra of the partially cross-linked pre-ceramic polymers.

Another advantage of this process is its high flexibility. Indeed, the composition, structure (amorphous or crystalline), microstructure, and properties of the ceramic material can be controlled and adjusted by varying many different parameters such as the composition and architecture of the pre-ceramic polymer, the amount and nature of the filler, the cross-linking step, and the pyrolysis parameters (atmosphere, heating rate, final temperature). Also, nonconventional heating systems such as laser or microwave heating or even athermal conversion processes like ion bombardment can be efficiently used. 

Since the mid 1990s, many research groups have concentrated on the development of fibers via the pyrolysis of appropriate pre-ceramic polymers [ 112]. In particular, the system Si-B-N-C has been of major interest due to the excellent high-temperature and oxidation stability of the resultant amorphous material [113]. In this case, the onset of crystallization may be as high as 1800 °C, while decomposition starts at 2000 °C in protective atmospheres. 

Polysiloxanes have also been pyrolyzed to give ceramics and organic/ inorganic hybrid materials. " Ihe general topic of pre-ceramic polymers is discussed chapter 9. ... 

A second approach for the generation of structured pre-ceramic polymer-based materials that will be briefly addressed here is based on the self-assembly of organic-inorganic core-shell particles, also referred to as colloidal crystallization. The self-assembly of almost monodisperse colloidal micro- and nanoparticles is a feasible method for gaining access to ceramic functional materials for various applications, especially if the final materials feature an optical band gap [234-238]. In general, colloidal crystals can be prepared from their dispersions by various techniques of deposition or spin coating, which are depicted in Fig. 3 [239, 240]. 

Mucalo, MJi., Milestone, N.B., and Brown, I.W.M. NMR and X-ray diffraction studies of amorphous and crystallized pyrolysis residues from pre-ceramic polymers.. /. Mater. Sci. 1997 32 2433-2444. 

Sato, K., Tezuka, A., Funayama, O., Isoda, T., Terada, Y., Kato, S. (1999). Fabrication and pressure testing of a gas-turbine component manufactured by a pre ceramic-polymer-impregnation method. Composites Science and Technology, 59, 853-859. doi 10.1016/80266-3538(99)00015-9. 

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